Controlling orbital-selective Kondo effects in a single molecule through coordination chemistry Noriyuki Tsukahara, Emi Minamitani, Yousoo Kim, Maki Kawai, and Noriaki Takagi Citation: The Journal of Chemical Physics 141, 054702 (2014); doi: 10.1063/1.4890654 View online: http://dx.doi.org/10.1063/1.4890654 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Understanding the Kondo resonance in the d-CoPc/Au(111) adsorption system J. Chem. Phys. 141, 084713 (2014); 10.1063/1.4893953 Spinorbit interaction detection using Kondo effect in single selfassembled InAs quantum dots AIP Conf. Proc. 1399, 355 (2011); 10.1063/1.3666400 Kondo effect in single cobalt phthalocyanine molecules adsorbed on Au(111) monoatomic steps J. Chem. Phys. 128, 234705 (2008); 10.1063/1.2940338 Tunable Orbital Pseudospin and Multilevel Kondo Effect in Carbon Nanotubes AIP Conf. Proc. 786, 482 (2005); 10.1063/1.2103914 Density-functional study of two Fe 4 -based single-molecule magnets J. Chem. Phys. 123, 044303 (2005); 10.1063/1.1961367
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THE JOURNAL OF CHEMICAL PHYSICS 141, 054702 (2014)
Controlling orbital-selective Kondo effects in a single molecule through coordination chemistry Noriyuki Tsukahara,1 Emi Minamitani,2,a) Yousoo Kim,2 Maki Kawai,1 and Noriaki Takagi1,b) 1
Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan 2 RIKEN, 2-1 Hirosawa, Saitama 351-0198, Japan
(Received 2 May 2014; accepted 24 June 2014; published online 1 August 2014) Iron(II) phthalocyanine (FePc) molecule causes novel Kondo effects derived from the unique electronic structure of multi-spins and multi-orbitals when attached to Au(111). Two unpaired electrons in the dz 2 and the degenerate dπ orbitals are screened stepwise, resulting in spin and spin+orbital Kondo effects, respectively. We investigated the impact on the Kondo effects of the coordination of CO and NO molecules to the Fe2+ ion as chemical stimuli by using scanning tunneling microscopy (STM) and density functional theory calculations. The impacts of the two diatomic molecules are different from each other as a result of the different electronic configurations. The coordination of CO converts the spin state from triplet to singlet, and then the Kondo effects completely disappear. In contrast, an unpaired electron survives in the molecular orbital composed of Fe dz 2 and NO 5σ and 2π * orbitals for the coordination of NO, causing a sharp Kondo resonance. The isotropic magnetic response of the peak indicates the origin is the spin Kondo effect. The diatomic molecules attached to the Fe2+ ion were easily detached by applying a pulsed voltage at the STM junction. These results demonstrate that the single molecule chemistry enables us to switch and control the spin and the many-body quantum states reversibly. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890654] I. INTRODUCTION
Controlling many-body quantum states of matter by external stimuli is of profound interest from the viewpoints of both fundamental science and technological application. For instance, the manipulation in a heavy fermion compound from antiferromagnetic to paramagnetic phases through the quantum critical point has been realized by tuning the magnetic field.1 The switching of superconductivity has been demonstrated in SrTiO3 thin films by changing the doping level with external electric field.2 Macroscopic magnetic and electric fields are utilized as external stimuli in these examples. Chemical reaction provides an alternative route. The introduction/removal of chemical species to/from functional center in molecular system as chemical stimuli can reversibly control the properties. Compared to the macroscopic fields, the higher selectivity and locality of chemical stimuli open up pathways to switch many-body quantum effect at singlemolecule level. It has been recently shown that the attachment of small molecules such as CO and NO to metalphthalocyanines or metal-porphyrins substantially alters their electronic and magnetic properties.3–10 When these magnetic molecules are attached to nonmagnetic metal substrates, the molecular spins behave as a magnetic impurity and the manybody interactions of the molecular spins with the substrate conduction electrons give rise to Kondo resonance state.11–17 a) Present address: Department of Materials Engineering, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
b) Electronic mail:
[email protected]
0021-9606/2014/141(5)/054702/9/$30.00
The response of the Kondo resonance to the coordination of small molecules is a prototypical phenomenon to explore the concept of chemical control of many-body quantum effect. However, the studies focusing on this point are still scarce. In the previous studies,15, 17 we found that iron phthalocyanine (denoted as FePc) molecule on Au(111) exhibits unique Kondo effects. The FePc molecule takes spin triplet (S = 1) in the gas phase. The localized spins in the dz 2 and doubly degenerate dzx /dyz (dπ ) orbitals of a central Fe2+ ion are aligned through Hund’s coupling to form the triplet spin state. Novel Kondo effects emerge reflecting the multi-spins and multi-orbitals of FePc, when the molecule is attached to the Au(111) surface. Resulting two Kondo resonances derived from the dz 2 and dπ orbitals are caused by different mechanisms. Since the dπ orbitals are degenerate, the localized electron occupying the dπ orbitals has both spin and orbital degrees of freedom, which are entangled to give rise to the spin+orbital Kondo effect. In contrast, the orbital degrees of freedom are not available for the Kondo effect in the dz 2 orbital so that the spin SU(2) Kondo effect takes place. These intriguing Kondo effects of FePc on Au(111) offer a novel opportunity to demonstrate the orbital-selective manipulation of the many-body quantum states through the coordination of small molecules. In this article, we demonstrate that the multi-orbital Kondo states in FePc on Au(111) can be selectively controlled by utilizing the coordination of CO or NO molecule as chemical stimuli. Using a combination of low-temperature scanning tunneling microscopy (STM) and density functional
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theory (DFT), we found that the two Kondo resonances of FePc on Au(111) completely disappear by the coordination of CO, whereas one of the Kondo resonances survives with the coordination of NO. The residual Kondo resonance in the NOcoordinated complex originates from the magnetic moments in the molecular orbital consisting of the Fe dz 2 orbital and the NO orbitals. The Kondo resonance shows the isotropic response to external magnetic field typical to the spin SU(2) Kondo effect. We successfully distill the multi-orbital Kondo resonances of FePc on Au(111) into typical spin Kondo resonance through the coordination chemistry.
J. Chem. Phys. 141, 054702 (2014)
3-layer Au slab with a vacuum of ∼15.6 Å thick along the surface normal. The positions of atoms in both coordinated complex and outermost two layers of Au slab were optimized without any constraint until the forces on individual atoms were less than 0.02 eV/Å. Because of the large dimensions of the supercell, the Brillouin zone was sampled with a single k-point only at point. We confirmed in the preliminary calculations that the results are not changed when increasing the number of k points to 3×3×1.
III. RESULTS AND DISCUSSION II. METHODS
All the STM experiments were performed at the sample temperature of 0.4–2.6 K in an ultrahigh-vacuum chamber (base pressure of 5 × 10−11 Torr). An electrochemically etched W wire was used as the STM tip. All the STM images were acquired by constant current mode with a bias voltage Vs applied to the sample. The first derivative of tunneling current I (dI/dVs ) was measured as a scanning tunneling spectroscopy (STS) spectrum with a lock-in technique where modulation voltage (Vmod = 0.06 mV and frequency f = 312.6 Hz) was overlapped to Vs with the feedback loop switched off. The evolution of Kondo resonance with magnetic field was measured by using two superconducting magnets which generate the magnetic fields parallel and perpendicular to the sample surface. The clean Au(111) surface was prepared by repeated cycles of Ar ion sputtering and annealing at 770 K. FePc molecules (95% purity, Tokyo Chemical Industry) were used without purification and were deposited by heating a cell at 600 K onto Au(111) that was kept at room temperature. Outgassing the FePc cell at 600 K in the UHV condition (
